In a hydraulic system wherein a variable speed pump having a speed sensitive unloading control is driven by a prime mover such as a diesel engine, it has been found that the characteristics of a diesel engine are such that the engine and pump will tend to produce an unstable system under certain conditions. Specifically, as the pump displacement is reduced when the torque is reduced, the engine will tend to operate at high speed which will drive the pump to full displacement so that a potentially unstable condition exists.
Among the objectives of the present invention are to provide a hydraulic system which obviates these problems and wherein the power is limited in response to speed of the prime mover which drives the hydraulic pump.
In accordance with the invention, the hydraulic system comprises a full flow supercharged variable displacement pump and a fixed flow charge pump adapted to be driven by the same prime mover as the variable displacement pump for supplying fluid to the variable displacement pump. An orifice is provided in the supply line from the charge pump to the variable displacement pump. A circuit is provided from a point downstream of the orifice to tank pressure, and a feedback valve is provided in series with the circuit. The feedback valve comprises a pair of mutually and oppositely adjustable flow restrictions in series with said circuit.
The restrictions are connected to the displacement varying means to vary them equally and oppositely in proportion to the movements of the displacement varying means. A speed sensing control valve is responsive to the pressure level between the two restrictions and the pressure level upstream of the orifice and is connected to the outlet of the variable displacement pump and operable to operate the displacement varying means to reduce the displacement of the pump when the pressure differential between the pressure at the outlet of the feedback means and the pressure upstream of the orifice falls below a predetermined value.